It is not unusual to hear about women, older people or racial/sexual minorities being under-represented in clinical trial populations, and there are policies in place (e.g., US National Institutes of Health (NIH) guidelines) to address these concerns.

Now, though, it appears that in some cases gay and lesbian patients are experiencing the same kind of marginalisation. An analysis by researchers at the Fox Chase Cancer Center in Philadelphia, US found that exclusion from clinical trials on grounds of sexual orientation was “not uncommon, particularly in studies with sexual function as an endpoint”.

As the NIH notes on its ClinicalTrials.gov database, inclusion or exclusion criteria based on factors such as age, gender, type and stage of disease, previous treatment history or other medical conditions are an important facet of trial design that helps to generate reliable results. But, it stresses, these criteria are not used to reject people personally but “to identify appropriate participants and keep them safe”.

As they describe in a letter to the New England Journal of Medicine, the Fox Chase team led by Dr Brian Egleston decided to look into the issue after encountering “proposed studies that explicitly excluded persons in same-sex relationships”.

They conducted exploratory searches of the ClinicalTrials.gov database to identify categories of studies that were likely to leave out lesbians and gay men. The team then looked for overt inclusion and exclusion criteria that would overtly restrict trials to heterosexual patients, such as a requirement that participants be in heterosexual relationships. The searches were restricted to studies with sites in the US.

Using search terms such as “couples”, “erectile dysfunction” and “hypoactive” (as in hypoactive sexual disorder), the researchers identified 243 studies, of which 37 (15%) included explicit exclusionary language. Industry-sponsored trials, multi-region studies and Phase III trials were most likely to exclude lesbians and gay men, while other variables, such as the age of the participants and whether the study accepted healthy volunteers, were not associated with exclusionary criteria.

The Fox Chase team also examined the eligibility criteria for 1,019 studies identified using the search term “asthma”. Exploratory searches indicated that trials in this category did not have high rates of exclusionary language – in fact, no asthma trials were found to exclude lesbians and gay men. But the researchers did find a clinical trial for attention deficit hyperactivity disorder that required participants to be “in a reciprocal relationship with the opposite sex”.

It is likely most gay and lesbian patients “are unaware that their sexual orientation is being used as a screening factor for participation in clinical trials”, the authors commented, adding: “Researchers should be held to careful scientific reasoning when they develop exclusion criteria that are based on sexual orientation”.